McElwain, J.C., Mayle, F.E. and Beerling, D.J. 2002. Stomatal evidence for a decline in atmospheric CO2 concentration during the Younger Dryas stadial: a comparison with Antarctic ice core records. Journal of Quaternary Science 17: 21-29.
What was done
The authors derived high-resolution (approximately 20- to 37-year accuracy) atmospheric CO2 histories from stomatal frequencies measured on subfossil leaves of Dryas integrifolia, Picea mariana, P. glauca and Larix laricina obtained from sediment cores extracted from two different sites in New Brunswick, Canada - Pine Ridge Pond (45°34'N, 67°06'W) and Splan Pond (45°15'N, 67°20'W) - that contained material spanning the period of time from approximately 13,000 to 10,500 years ago.
What was learned
The data revealed an abrupt drop in atmospheric CO2 concentration of approximately 75 ppm at the onset of the Younger Dryas cold event. This drop in CO2 lagged the event-defining temperature drop by approximately 130 years. Then, at the end of the Younger Dryas, there was a rapid rise in atmospheric CO2 concentration that was (within the time-resolution error bounds of the data) essentially coeval with the increase in temperature that brought an end to the Younger Dryas and initiated the Holocene. In absolute terms, the pre-Younger Dryas CO2 concentration was something on the order of 300 to 320 ppm, the concentration during the Younger Dryas interval approximately 235 ppm, and the concentration immediately afterwards somewhere in the range of 285 to 300 ppm.
In comparing their results with atmospheric CO2 concentrations derived from polar ice core data, the authors noted that the best such data available had time resolutions on the order of 200 to 550 years. Degrading (averaging) their data accordingly, they were able to closely match the ice core results (a steady increase in atmospheric CO2 concentration from the beginning to the end of the Younger Dryas interval). But there was no way the ice core data could mimic the much-finer-scale story told by their data, including the dramatic decline in atmospheric CO2 concentration at the inception of the Younger Dryas and the dramatic increase in the air's CO2 content at the conclusion of the cold event.
What it means
The observation that the decline in atmospheric CO2 concentration at the onset of the Younger Dryas lagged the decline in temperature by some 130 years clearly demonstrates that the change in the air's CO2 content did not cause the change in temperature, but that the temperature drop - or whatever caused it - was responsible for the decline in CO2 concentration. Hence, there is no reason to believe that the same was not true at the end of the Younger Dryas, although even the authors' good data resolution was not good enough to explicitly demonstrate that fact. Nevertheless, it is demonstrated for a number of glacial-to-interglacial transitions described on our website (see many of the Journal Reviews and Editorials filed under CO2-Temperature Correlations in our Subject Index).
Clearly, the scientific community is gingerly edging its way toward the conclusion we have long espoused on this subject, i.e., that temperature changes drive changes in the air's CO2 content and not vice versa.
Reviewed 1 May 2002